Because a lithium ion secondary battery has a high energy density, the lithium ion secondary battery attracts attention as a battery used for vehicle mounting such as a railroad and a vehicle or for storing power generated by photovoltaic power generation and wind power generation and supplying the power to a power system. As examples of the vehicle mounted with the lithium ion secondary battery (hereinafter, appropriately referred to as a “battery”), there are a zero-emission electric vehicle not mounted with an engine, a hybrid electric vehicle mounted with both the engine and a secondary battery, and a plug-in hybrid electric vehicle performing direct charging from a system power supply. In addition, the lithium ion secondary battery is expected as a stationary power storage system to supply power in an emergency situation where a power system is shut down.
For the various uses described above, a battery having a high capacity and a long life is required. For example, it is required that a decrease rate of a capacity of a rechargeable battery, that is, a decrease rate of a battery capacity is low and a maintenance rate of the battery capacity is high over a long period, even when an environment temperature increases or a charge/discharge cycle is repeated. In addition, a storage characteristic and a cycle life in an environment of a high temperature of 60° C. or more become important requirement performance, due to heat radiation from a road surface or heat conduction from the interior of the vehicle.
At the present time, in graphite generally used as a material of an electrode, a capacity reaches a capacity close to an approximate theoretical capacity and it is not anticipated that the capacity of the battery is further increased. For this reason, using a Si-based material as a material of an electrode is examined from the viewpoint of the capacity increase of the battery. However, it is known that Si has large expansion/contract according to charge/discharge and it is likely to cause cycle deterioration by repeating the charge/discharge.
In view of the above circumstances, PTL 1 discloses a non-aqueous secondary battery in which, when a sum of SiOx and graphite is set to 100 wt % in a negative electrode mixture layer, a ratio of SiOx is 2 to 30 wt % and first charge/discharge efficiency of a negative electrode is higher than first charge/discharge efficiency of a positive electrode.
In addition, PTL 2 discloses a life estimation method and a degradation control method for a lithium ion secondary battery in which a voltage at the time of an open circuit after discharge in the lithium ion secondary battery when charge/discharge is performed by a different cycle number is detected at least two times, according to the passage of a charge/discharge cycle. In PTL 2, at least two of individual detected voltage values are plotted for individual cycle numbers, a circular arc passing each plotted point is drawn, and a life of the lithium ion secondary battery is estimated on the basis of the magnitude of the circular arc. In PTL 2, advancement of degradation can be suppressed by controlling charge and discharge of the lithium ion secondary battery, on the basis of the estimated life.
In addition, PTL 3 discloses a discharge control method for a non-aqueous electrolyte secondary battery that is a method of discharging a non-aqueous electrolyte secondary battery using silicon oxide containing lithium as a negative electrode active material and executes control to discharge the non-aqueous electrolyte secondary battery in a range in which a negative electrode voltage for a lithium reference electrode does not exceed 0.6 V.